Pseudo constant current multiple cell battery charger configured with a parallel topology

ABSTRACT

A multiple cell battery charger configured in a parallel topology provides constant current charging. The multiple cell battery charger requires fewer active components than known serial battery chargers, while at the same time preventing a thermal runaway condition. The multiple cell battery charger in accordance with the present invention is a constant voltage constant current battery charger that includes a regulator for providing a regulated source of direct current (DC) voltage to the battery cells to be charged. The battery charger also includes a pair of battery terminals coupled in series with a switching device, such as a field effect transistor (FET) and optionally a battery cell charging current sensing element, forming a charging circuit. In a charging mode, the serially connected FET conducts, thus enabling the battery cell to be charged. The FETs are controlled by a microprocessor that monitors the battery cell voltage and cell charging current and optionally the cell temperature. The microprocessor periodically adjusts the charging current of each cell to maintain a relatively constant current. When the microprocessor senses a voltage or temperature indicative that the battery cell is fully charged, the FET is turned off, thus disconnecting the battery cell from the circuit. Accordingly, the battery charger in accordance with the present invention utilizes fewer active components and is thus less expensive to manufacture than known battery chargers configured with a serial topography while at the same time providing constant current charging to avoid a thermal runaway condition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of commonly owned copendingU.S. patent application Ser. No. 10/863,920, filed on Jun. 9, 2004,entitled “Multiple Cell Battery Charger Configured with a ParallelTopology”, attorney docket no. 211552-00053.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a. battery charger and moreparticularly, to a battery charger for charging two or more rechargeablebattery cells using a parallel battery charger topology which providesconstant current charging.

2. Description of the Prior Art

Various portable devices and appliances are known to use multiplerechargeable battery cells, such as AA and AAA battery cells. In orderto facilitate charging of the battery cells for such multiple cellappliances, multiple cell battery chargers have been developed. Bothparallel and series topologies are known for such multiple cell batterychargers. For example, U.S. Pat. Nos. 5,821,733 and 6,580,249, as wellas published U.S. Patent Application U.S. 2003/0160593, disclosemultiple cell battery chargers configured in a series topology. U.S.Pat. Nos. 6,034,506 and 6,586,909 as well as published U.S. PatentApplication U.S. 2003/0117109 A1 disclose battery chargers configured ina parallel topology.

In multiple cell battery chargers configured in a series topology, aseries charging current is applied to a plurality of serially coupledbattery cells. Because the internal resistance and charge on theindividual cells may vary during charging, it is necessary with suchbattery chargers to monitor the voltage across and/or temperature ofeach cell in order to avoid overcharging any of the serially connectedcells. In the event that an over-voltage condition is sensed, it isnecessary to shunt charging current around the cell to preventovercharging of any of the individual serially connected cells. Thus,such multiple cell battery chargers normally include a parallel shuntaround each of the serially connected cells. As such, when a batterycell becomes fully charged, additional charging current is thus shuntedaround the cell to prevent overcharging and possible damage to the cell.In addition, it is necessary to prevent discharge of such seriallyconnected battery cells when such cells are not being charged.

Various embodiments of a multiple cell battery charger configured with aserial charging topography are disclosed in the '733 patent. In oneembodiment, a Zener diode is connected in parallel across each of theserially connected battery cells. The Zener diode is selected so thatits breakdown voltage is essentially equivalent to the fully-chargedvoltage of the battery cell. Thus, when any of the cells become fullycharged, the Zener diode conducts and shunts current around that cell toprevent further charging of the battery cell. Unfortunately, the Zenerdiode does not provide relatively accurate control of the switchingvoltage.

In an alternate embodiment of the battery charger disclosed in the '733patent, a multiple cell battery charger with a series topology isdisclosed in which a field effect transistors (FET) are used in place ofthe Zener diodes to shunt current around the battery cells. In thatembodiment, the voltage across each of the serially connected cells ismonitored. When the voltage measurements indicate that the cell is fullycharged, the FET is turned on to shunt additional charging currentaround the fully charged cell. In order to prevent discharge of batterycells, isolation switches, formed from additional FETs, are used. Theseisolation switches simply disconnect the charging circuit from theindividual battery cells during a condition when the cells are not beingcharged.

U.S. Pat. No. 6,580,249 and published U.S. Patent Application No. U.S.2003/01605393 A1 also disclosed multiple cell battery chargersconfigured in a serial topology. The multiple cell battery chargersdisclosed in these publications also include a shunt device, connectedin parallel around each of the serially coupled battery cells. In theseembodiments, FETs are used for the shunts. The FETs are under thecontrol of a microprocessor. Essentially, the microprocessor monitorsthe voltage and temperature of each of the serially connected cells.When the microprocessor senses that the cell voltage or temperature ofany cell is above a predetermined threshold indicative that the cell isfully charged, the microprocessor turns on the FET, thus shuntingcharging current around that particular battery cell. In order toprevent discharge of the serially connected cells when no power isapplied to the battery charger, blocking devices, such as diodes, areused.

Although such multiple cell battery chargers configured in a seriestopology are able to simultaneously charge multiple battery cellswithout damage, such battery chargers are as discussed above, notwithout problems. For example, such multiple cell battery chargersrequire at least two active components, namely, either a Zener diode ora FET as a shunt and either a FET or diode for isolation to preventdischarge. The need for at least two active devices drives up the costof such multiple battery cell chargers.

In order to avoid the problems associated with multiple cell seriesbattery chargers, multiple cell battery chargers configured in aparallel topology are known to be used. Examples of such parallelchargers are disclosed in U.S. Pat. Nos. 6,034,506 and 6,586,909, aswell as U.S. Published Patent Application No. U.S. 2003/0117109. U.S.Pat. No. 6,586,909 and published U.S. Application No. U.S. 2003/0117109discloses a multiple cell battery chargers for use in chargingindustrial high capacity electrochemical batteries. These publicationsdisclose the use of a transformer having a single primary and multiplebalanced secondary windings that are magnetically coupled together byway of an induction core. Each battery cell is charged by way of aregulator, coupled to one of the multiple secondary windings. While sucha configuration may be suitable for large industrial applications, it ispractically not suitable for use in charging appliance size batteries,such as, AA and AAA batteries.

Finally, U.S. Pat. No. 6,034,506 discloses a multiple cell batterycharger for charging multiple lithium ion cells in parallel. As shownbest in FIG. 3 of the '506 patent, a plurality of serially connectedlithium ion battery cells are connected together forming a module.Multiple modules are connected in series and in parallel as shown inFIG. 2 of the '506 patent. Three isolation devices are required for eachcell making the topology disclosed in the '506 patent even moreexpensive to manufacture than the series battery chargers discussedabove.

Another problem associated with parallel battery chargers is thermalrunaway. In particular, it is known parallel battery chargers provideconstant potential charging. With such constant potential charging, asthe cell voltage increases, the temperature and charging current of thecell also increase. Continued constant potential charging of the batterycell causes the current to continue to rise as well as the rate ofchange of the temperature to increase significantly, resulting in athermal runaway condition. Thus, there is a need for a battery chargerwhich requires fewer active components than known battery chargers andis thus less expensive to manufacture and also avoids a thermal runawaycondition.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a multiple cell batterycharger configured in a parallel topology which provides constantcurrent charging. The multiple cell battery charger requires feweractive components than known serial battery chargers, while at the sametime preventing a thermal runaway condition. The multiple cell batterycharger in accordance with the present invention is a constant voltageconstant current battery charger that includes a regulator for providinga regulated source of direct current (DC) voltage to the battery cellsto be charged. The battery charger also includes a plurality of chargingcircuits, each charging circuit including a pair of battery terminalscoupled in series with a switching device, such as a field effecttransistor (FET) and optionally a battery cell charging current sensingelement. In a charging mode, the serially connected FET conducts, thusenabling the battery cell to be charged. The FETs are controlled by amicroprocessor that monitors the battery cell voltage and cell chargingcurrent and optionally the cell temperature. The microprocessorperiodically adjusts the charging current of each cell by turning theFETs of the respective charging circuits off to maintain relativelyconstant charge (i.e. ampere-sec.) to the various cells during each PWMcycle of the regulator, thus avoiding a thermal runaway condition. Themicroprocessor also senses the voltage and optionally the temperature ofeach cell. When the microprocessor determines that the battery cell isfully charged, the FET is turned off, thus disconnecting the batterycell from the circuit. Accordingly, the battery charger in accordancewith the present invention utilizes fewer active components and is thusless expensive to manufacture than known battery chargers configuredwith a serial topography while at the same time providing constantcurrent charging to avoid a thermal runaway condition.

DESCRIPTION OF THE DRAWING

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1 is a schematic diagram of the battery charger in accordance withthe present invention.

FIG. 2 is a graphical illustration of the voltage, pressure, and/ortemperature charging characteristics as a function of time as anexemplary NiMH battery.

FIGS. 3A-3E illustrate exemplary flow-charts for the battery charger forthe present invention.

FIGS. 4A-4D illustrate the charging current of four exemplary cells atdifferent time periods during an exemplary charging cycle.

FIG. 5 is an exemplary flow chart which illustrates a constant currentmode of operation.

DETAILED DESCRIPTION

The present invention relates to a constant voltage constant currentmultiple cell battery charger configured in a parallel topology that isadapted to charge multiple battery cells connected in parallel. Theconstant current mode of operation is illustrated and described inconnection with FIGS. 4 and 5.

Power Supply and Regulator

The battery charger, generally identified with the reference 20,includes a power supply 22 and a regulator 24. In an AC application, thepower supply 22 is configured to receive a source of AC power, such as120 volts AC, and convert it to a non-regulated source of DC power byway of a bridge rectifier (not shown), for example or other device, suchas a switched mode power supply. In DC applications, the power supply 22may simply be a unregulated source of DC, for example in the range of 10to 16 volts DC, such as a vehicular power adapter from an automobile.The unregulated source of DC power from the power supply 22 may beapplied to, for example, to a regulator, such as, a DC buck regulator24, which generates a regulated source of DC power, which, in turn, isapplied to the battery cells to be charged.

The regulator 24 may be an integrated circuit (IC) or formed fromdiscrete components. The regulator 24 may be, for example, a switchingtype regulator which generates a pulse width modulated (PWM) signal atits output. The regulator 24 may be a synchronous buck regulator 24, forexample, a Linear Technology Model No. LTC 1736, a FairchildSemiconductor Model No. RC5057; a Fairchild Semiconductor Model No.FAN5234; or a Linear Technology Model No. LTC1709-85 or others.

The output of the regulator 24 may optionally be controlled by way of afeedback loop. In particular, a total charging current sensing device,such as a sensing resistor R11, may be serially coupled to the output ofthe regulator 24. The sensing resistor R11 may be used to measure thetotal charging current supplied by the regulator 24. The value of thetotal charging current may be dropped across the sensing resistor R11and sensed by a microprocessor 26. The microprocessor 26 may beprogrammed to control the regulator 24, as will be discussed in moredetail below, to control the regulator 24 based on the state of chargeof the battery cells being charged.

Battery Charger Schematic

As shown in FIG. 1, the battery charger 20 may optionally be configuredto charge four battery cells 28, 30, 32, and 34. As shown, these batterycells 28, 30, 32 and 34 are electrically coupled to corresponding pairsof battery terminals: T₁ and T₂; T₃ and T₄; T₅ and T₆; and T₇ and T₈,respectively. However, the principles of the present invention areapplicable to two or more battery cells.

Each battery cell 28, 30, 32 and 34 is serially connected to a switchingdevice, such as a field effect transistor (FET) Q12, Q13, Q14 and Q15.More particularly, the source and drain terminals of each of the FETsQ12, Q13, Q14 and Q15 are serially connected to the battery cells 28,30, 32 and 34. In order to sense the charging current supplied to eachof the battery cells 28, 30, 32 and 34, a current sensing devices, suchas the sensing resistors R37, R45, R53, R60, may be serially coupled tothe serial combination of the FETs Q12, Q13, Q14 and Q15 ; and the pairsof battery terminals, T₁; and T₂; T₃ and T₄; T₅ and T₆; and T₇ and T₈,The serial combination of the battery terminals T₁ and T₂; T₃ and T₄;T₅and T₆; and T₇ and T₈; FETs Q12, Q12, Q14 and Q15; and the optionalcharging current sensing devices R37, R45, R53 and R60, respectively,form a charging circuit for each battery cell 28, 30, 32 and 34. Thesecharging circuits, in turn, are connected together in parallel.

The charging current supplied to each of the battery cells 28, 30, 32and 34 can vary due to the differences in charge, as well as theinternal resistance of the circuit and the various battery cells 28, 30,32, and 34. This charging current as well as the cell voltage andoptionally the cell temperature may be sensed by the microprocessor 26.In accordance with an important aspect of the present invention, themultiple cell battery charger 20 may be configured to optionally sensethe charging current and cell voltage of each of the battery cells 28,30, 32 and 34, separately. This may be done by control of the seriallyconnected FETS Q12, Q13, Q14 and Q15. For example, in order to measurethe cell voltage of an individual cell, such as the cell 28, the FET Q12is turned on while the FETs Q13, Q14 and Q15 are turned off. When theFET 12 is turned on, the anode of the cell 28 is connected to systemground. The cathode of the cell is connected to the V_(sen) terminal ofthe microprocessor 26. The cell voltage is thus sensed at the terminalV_(sen).

As discussed above, the regulator 24 may be controlled by themicroprocessor 26. In particular, the magnitude of the total chargingcurrent supplied to the battery cells 28, 30, 32 and 34 may be used todetermine the pulse width of the switched regulator circuit 24. Moreparticularly, as mentioned above, the sensing resistor R11 may be usedto sense the total charging current from the regulator 24. Inparticular, the charging current is dropped across the sensing resistorR11 to generate a voltage that is read by the microprocessor 26. Thischarging current may be used to control the regulator 24 andspecifically the pulse width of the output pulse of the pulse widthmodulated signal forming a closed feedback loop. In another embodimentof the invention, the amount of charging current applied to theindividual cells Q12, Q13, Q14 and Q15 may be sensed by way of therespective sensing resistors R37, R45, R53 and R60 and used for controlof the regulator 24 either by itself or in combination with the totaloutput current from the regulator 24. In other embodiments of theinvention, the charging current to one or more of the battery cells 28,30, 32 and 34 may be used for control.

In operation, during a charging mode, the pulse width of the regulator24 is set to an initial value. Due to the differences in internalresistance and state of charge of each of the battery cells 28, 30, 32and 34 at any given time, any individual cells which reach their fullycharged state, as indicated by its respective cell voltage, as measuredby the microprocessor 26. More particularly, when the microprocessor 26senses that any of the battery cells 28, 30, 32 or 34 are fully charged,the microprocessor 26 drives the respective FETs Q12, Q13, Q14, or Q15open in order to disconnect the respective battery cell 28, 30, 32 and34 from the circuit. Since the battery cells are actually disconnectedfrom the circuit, no additional active devices are required to protectthe cells 28, 30, 32, and 34 from discharge. Thus, a single activedevice per cell (i.e., FETs Q12, Q13, Q14 and Q15) are used in place oftwo active devices normally used in multiple cell battery chargersconfigured with a serial topology to provide the dual function ofpreventing overcharge to individual cells and at the same timeprotecting those cells from discharge.

As mentioned above, the charging current of each of the battery cells28, 30, 32, and 34 is dropped across a sensing resistor R37, R45, R53,and R60. This voltage may be scaled by way of a voltage divider circuit,which may include a plurality of resistors R30, R31, R33 and R34, R35,R38, R39, R41, R43, R44, R46, R48, R49, R51, R52, R54, R57, R58, R59,R61, as well as a plurality of operational amplifiers U4A, U4B, U4C andU4D. For brevity, only the amplifier circuit for the battery cell 28 isdescribed. The other amplifier circuits operate in a similar manner. Inparticular, for the battery cell 28, the charging current through thebattery cell 28 is dropped across the resistor R37. That voltage drop isapplied across a non-inverting input and inverting input of theoperational amplifier U4D.

The resistors R31, R33, R34, and R35 and the operational amplifier U4Dform a current amplifier. In order to eliminate the off-set voltage, thevalue of the resistors R33 and R31 value are selected to be the same andthe values of the resistors R34 and R35 value are also selected to bethe same. The output voltage of the operational amplifier U4D=voltagedrop across the resistor R37 multiplied by the quotient of the resistorvalue R31 resistance value divided by the resistor value R34. Theamplified signal at the output of the operational amplifier U4D isapplied to the microprocessor 26 by way of the resistor R30. Theamplifier circuits for the other battery cells 30, 32, and 34 operate ina similar manner.

Charge Termination Techniques

The battery charger in accordance with the present invention canimplement various charge termination techniques, such as temperature,pressure, negative delta, and peak cut-out techniques. These techniquescan be implemented relatively easily by program control and are bestunderstood with reference to FIG. 2. For example, as shown, threedifferent characteristics as a function of time are shown for anexemplary nickel metal hydride (NiMH) battery cell during charging. Inparticular, the curve 40 illustrates the cell voltage as a function oftime. The curves 42 and 44 illustrate the pressure and temperaturecharacteristics, respectively, of a NiMH battery cell under charge as afunction of time.

In addition to the charge termination techniques mentioned above,various other charge termination techniques the principles of theinvention are applicable to other charge termination techniques as well.For example, a peak cut-out charge termination technique, for example,as described and illustrated in U.S. Pat. No. 5,519,302, herebyincorporated by reference, can also be implemented. Other chargetermination techniques are also suitable.

FIG. 2 illustrates an exemplary characteristic curve for an exemplaryNiMH or NiCd battery showing the relationship among current, voltage andtemperature during charge. More particularly, the curve 40 illustratesthe cell voltage of an exemplary battery cell under charge. In responseto a constant charge, the battery cell voltage, as indicated by thecurve 40, steadily increases over time until a peak voltage valueV_(peak) is reached as shown. As illustrated by the curve 44, thetemperature of the battery cell under charge also increases as afunction of time. After the battery cell reaches its peak voltageV_(peak), continued charging at the increased temperature causes thebattery cell voltage to drop. This drop in cell voltage can be detectedand used as an indication that the battery's cell is fully charged. Thischarge termination technique is known as the negative delta V technique.

As discussed above, other known charge termination techniques are basedon pressure and temperature. These charge termination techniques relyupon physical characteristics of the battery cell during charging. Thesecharge termination techniques are best understood with respect to FIG.2. In particular, the characteristic curve 42 illustrates the internalpressure of a NiMH battery cell during charging while the curve 44indicates the temperature of a NiMH battery cell during testing. Thepressure-based charge termination technique is adapted to be used withbattery cells with internal pressure switches, such as the Rayovacin-cell charge control (I-C³ )¹, NiMH battery cells, which have aninternal pressure switch coupled to one or the other anode or cathode ofthe battery cell. With such a battery cell ,as the pressure of the cellbuilds up due to continued charging, the internal pressure switch opens,thus disconnecting the battery cell from the charger.(I-C³ ) is a trademark of the Rayovac Corporation.

Temperature can also be used as a charge termination technique. Asillustrated by the characteristic curve 44, the temperature increasesrather gradually. After a predetermined time period, the slope of thetemperature curve becomes relatively steep. This slope, dT/dt may beused as a method for terminating battery charge.

The battery charge in accordance with the present invention can alsoutilize other known charge termination techniques. For example, in U.S.Pat. No. 5,519,302 discloses a peak cut-out charge termination techniquein which the battery voltage and temperature is sensed. With thistechnique, a load is attached to the battery during charging. Thebattery charging is terminated when the peak voltage is reached andreactivated as a function of the temperature.

Battery Charger Software Control

FIGS. 3A-3E illustrate exemplary flow-charts for controlling the batterycharger in accordance with the present invention. Referring to the mainprogram, as illustrated in FIG. 3A, the main program is started uponpower-up of the microprocessor 26 in step 50. Upon power-up, themicroprocessor 26 initializes various registers and closes all of theFETs Q12, Q13, Q14, and Q15 in step 52. The microprocessor 26 also setsthe pulse-width of the PWM output of the regulated 24 to a nominalvalue. After the system is initialized in step 52, the voltages acrossthe current sensing resistors R37, R45, R53, and R60 are sensed todetermine if any battery cells are currently in any of the pockets instep 54. If the battery cell is detected in one of the pockets, thesystem control proceeds to step 56 in which the duty cycle of the PWMout-put of the regulator 24 is set. In step 58, a charging mode isdetermined. After the charging mode is determined, the microprocessor 26takes control of the various pockets in step 60 and loops back to step54.

A more detailed flow-chart is illustrated in FIG. 3B. Initially, in step50, the system is started upon power-up of the microprocessor 26. Onstart-up, the system is initialized in step 52, as discussed above. Asmentioned above, the battery charger in accordance with the presentinvention includes two or more parallel connected charging circuits.Each of the charging circuits includes a switching device, such as aMOSFETs Q12, Q13, Q14, or Q15, serially coupled to the batteryterminals. As such, each charging circuit may be controlled by turningthe MOSFETs on or off, as indicated in step 66 and discussed in moredetail below. In step 68, the output voltage and current of theregulator 24 is adjusted to a nominal value by the microprocessor 26.After the regulator output is adjusted, a state of the battery cell ischecked in step 70. As mentioned above, various charge terminationtechniques can be used with the present invention. Subsequent to step70, the charging current is detected in step 72 by measuring thecharging current dropped across the current sensing resistors R37, R45,R53, or R60.

One or more temperature based charge termination techniques may beimplemented. If so, a thermistor may be provided to measure the externaltemperature of the battery cell. One such technique is based on dT/dt.Another technique relates to temperature cutoff. If one or more of thetemperature based techniques are implemented, the temperature ismeasured in step 74. If a dT/dt charge termination technique isutilized, the temperature is taken along various points along the curve44 (FIG. 2) to determine the slope of the curve. When the slope isgreater than a predetermined threshold, the FET for that cell is turnedoff in step 76.

As mentioned above, the system may optionally be provided with negativedelta V charge termination. Thus, in step 78, the system may constantlymonitor the cell voltage by turning off all but one of the switchingdevices Q12, Q13, Q14, and Q15 and measuring the cell voltage along thecurve 40 (FIG. 2). When the system detects a drop in cell voltagerelative to the peak voltage V_(sen), the system loops back to step 66to turn off the switching device Q12, Q13, Q14, and Q15 for that batterycell.

As mentioned above, a temperature cut-off (TCO) charge terminationtechnique may be implemented. This charge termination technique requiresthat the temperature of the cells 28, 30, 32 and 34 to be periodicallymonitored. Should the temperature of any the cells 28, 30, 32 and 34exceed a predetermined value, the FET for that cell is turned off instep 80. In step 82, the charging time of the cells 28, 30, 32, and 34is individually monitored. When the charging time exceeds apredetermined value, the FET for that cell is turned off in step 82. ALED indication may be provided in step 84 indicating that the battery isbeing charged.

FIG. 3C illustrates a subroutine for charging mode detection. Thissubroutine may be used to optionally indicate whether the batterycharger 20 is in a “no-cell” mode; “main-charge” mode;“maintenance-charge” mode; an “active” mode; or a “fault” mode. Thissubroutine corresponds to the block 58 in FIG. 3A. The system executesthe charging mode detection subroutine for each cell being charged.Initially, the system checks in step 86 the open-circuit voltage of thebattery cell by checking the voltage at terminal V_(sen) of themicroprocessor 26. If the open-circuit voltage is greater than or equalto a predetermined voltage, for example, 2.50 volts, the system assumesthat no battery cell is in the pocket, as indicated in step 88. If theopen-circuit voltage is not greater than 2.50 volts, the system proceedsto step 90 and checks whether the open-circuit voltage is less than, forexample, 1.90 volts. If the open circuit voltage is not less than 1.90volts, the system indicates a fault mode in step 92. If the open-circuitvoltage is less than 1.90 volts, the system proceeds to step 94 andchecks whether the open-circuit voltage is less than, for example, 0.25volts. If so, the system returns an indication that the battery chargeris in inactive mode in step 96. If the open-circuit voltage is not lessthan, for example, 0.25 volts, the system proceeds to step 98 and checkswhether a back-up timer, is greater than or equal to, for example, twominutes. If not, the system returns an indication that battery charger20 is in the active mode in step 96. If the more than, for example, twominutes has elapsed, the system checks in step 100 whether the batterycell voltage has decreased more than a predetermined value, for example,6.2 millivolts. If so, the system returns an indication in step 102 of amaintenance mode. If not, the system proceeds to step 104 and determineswhether the back-up timer is greater or equal to a maintenance timeperiod, such as two hours. If not, the system returns an indication instep 106 of a main charge mode. If more than two hours, for example haselapsed, the system returns an indication in step 102 of a maintenancemode.

FIG. 3D illustrates a subroutine for the PWM duty cycle control. Thissubroutine corresponds to block 56 in FIG. 3A. This subroutine initiallychecks whether or not a cell is present in the pocket in step 108 asindicated above. If there is no cell in the pocket, the duty cycle ofthe PWM is set to zero in step 110. When there is a battery cell beingcharged, the PWM output current of the regulator 24 is sensed by themicroprocessor 26 by way of sensing resistor R11. The microprocessor 26uses the output current of the regulator 24 to control the PWM dutycycle of the regulator 24. Since the total output current from theregulator 24 is dropped across the resistor R11, the system checks instep 111 whether the voltage V_(sen) is greater than a predeterminedvalue, for example, 2.50 volts in step 111. If so, the PWM duty cycle isdecreased in step 115. If not, the system checks whether the totalcharging current for four pockets equal a predetermined value. If so,the system returns to the main program. If not, the system checks instep 114 whether the charging current is less than a preset value. Ifnot, the PWM duty cycle is decreased in step 115. If so, the PWM dutycycle is increased in step 116.

The pocket on-off subroutine is illustrated in FIG. 3E. This subroutinecorresponds to the block 60 in FIG. 3A. Initially, the system checks instep 118 whether the battery cell in the first pocket (i.e. channel 1)has been fully charged. If not, the system continues in the main programin FIG. 3A, as discussed above. If so, the system checks in step 120which channels (i.e pockets) are charging in order to take appropriateaction. For example, if channel 1 and channel 2 are charging and channel3 and channel 4 are not charging, the system moves to step 122 and turnsoff channel 3 and channel 4, by turning off the switching devices Q14and Q15 and moves to step 124 and turns on channel 1 and channel 2, byturning on the switching device Q12 and Q13.

The channels refer to the individual charging circuits which include theswitching devices Q12, Q13, Q14, and Q15. The channels are controlled byway of the switching devices Q12, Q13, Q14 or Q15 being turned on or offby the microprocessor 26.

Psuedo-Constant Current Operation

Referring first to FIG. 2, the curve 45 illustrates that the charge ismaintained as relatively constant over the charging cycle of a batterycell. This charging technique is best understood with reference to FIGS.4A-4D, which illustrate an exemplary charging of an exemplary four (4)cell charger during an exemplary charging cycle (i.e. PWM cycle of theregulator 24). In particular, FIG. 4A illustrates an exemplary chargingcondition for a battery cell 28 (FIG. 1) connected to terminals T1-T2.FIG. 4B illustrates an exemplary charging condition for a battery cell30 connected to terminals T3-T4. FIG. 4B illustrates an exemplarycharging condition for a battery cell 32 connected to terminals T5-T6.Finally, FIG. 4D illustrates an exemplary charging condition for abattery cell 34 connected to terminals T7-T8.

By controlling the charging time and charge applied to the battery cells28, 30, 32, 34, the average charge applied to each battery cell 28, 30,32, 34 can be maintained to be relatively constant, which, in essence,creates a constant current condition (“pseudo constant current”condition), thus avoiding a thermal runaway condition. The chargeapplied to each battery cell 28, 30, 32, 34 in ampere-seconds is theproduct of the current and the charging time. Graphically, the chargesupplied to each battery cell 28, 30, 32, 34 is the area under thecurves illustrated in FIGS. 4A-4D. For example, with reference to FIG.4A, the charge is the area within the shaded box, identified as Q1. Thearea Q1 is the product of the charging current c1 applied to the batterycell 28 and the time t1 that the charging current c1 was applied. Withreference to FIG. 1, the total current available from the power supply22 and regulator 24 is fixed. Assuming that the total charging currentavailable from the regulator 24(FIG. 1), is for example 4.0 amperes, atany given time during a charging cycle, the currentI_(a)+I_(b)+I_(c)+I_(d)=4.0 amperes. Now referring to FIGS. 4A-4D andassuming initially that the charging currents applied to the batterycells 28, 30, 32 and 34 are: I_(a)=1.4 amperes; I_(b)=1.2 amperes;I_(c)=0.8 amperes; and I_(d)=0.6 amperes, respectively.. Assuming thatthe charge applied to each cell 28, 30, 32, 34 during each chargingcycle is selected to be 1.0 ampere-seconds per charging cycle, then theFET Q12 is controlled to disconnect the battery cell 28 after 0.7143seconds at a current of 1.4 amperes (0.7143×1.40=1.0 coulombs). Thus,FIG. 4A illustrates that the cell 28 is cut off after time t₁ or, inthis example, 0.7143 seconds in the first charging cycle.

After the charging current to the first battery cell 28 is cutoff, thetotal charging current, for example, 4.0 amperes will be available tothe three remaining battery cells 30, 32, and 34. Thus, after the FETQ12 is opened, the charging current to the battery cells 30, 32, and 34is measured. Assume that the charging current is I_(a)=0 amperes,I_(b)=1.85 amperes, I_(c)=1.23 amperes, and I_(d)=0.992 amperes for thebattery cells 28, 30, 32 and 34, respectively.

The termination time for the battery cell 30 is best understood withreference to FIG. 4B. In this example, the charge supplied to thebattery cell 30 up to time t₁ is represented by the area under the curverepresented by the shaded areas Q2 and Q3 in FIG. 4B. The box Q2 refersto the charge during the first time period t₁ in which the chargingcurrent was supplied to all four battery cells 28, 30, 32, and 34. Usingthe example above, t₁ was assumed to be 0.7143 seconds and the chargingcurrent I_(b) supplied to the battery cell 30 during this time wasassumed to be 1.2 amps. Thus, in order. for the total charge supplied tothe battery cell 30 to be 1.0 ampere-seconds (coulomb), Q2+Q3 must beequal to 1.0 ampere-seconds. In other words, assuming the valuesdiscussed above, t₂−=1.0−(1.2×0.7143)/1.85. Solving the equation yields,t₂=0.077 seconds. As such the microprocessor 26 will turn off the FETQ13 (FIG. 1) time t₂, thus connecting the battery cell 30 from thecircuit. After the battery cells 28 and 30 have been disconnected fromthe circuit, the current applied to the battery cells 30 and 32 ismeasured. At this stage, I_(a)=I_(b)=0, and I_(c)+I_(d)=4 amps.

Assume I_(c)=2.29 amps and I_(d)=1.71 amps. Referring to FIG. 4C, inorder for the total ampere seconds applied to the battery cell 32 to beequal to 1.0 ampere seconds, the area in the boxes Q4 ,Q5 and Q6 musttotal 1.0 ampere seconds. During time t₁ (i.e. 0.7143 seconds), thebattery cell 32 was assumed to be charged with a charging current of 0.8amps. Thus, Q4=0.7143×0.8=0.57144 ampere-seconds. During the time periodbetween t₁ and t₂, the charging current I_(c) applied to the batterycell 32 is assumed to be 1.23 amperes for a time period equal to 0.077seconds. Thus, Q₅=1.23×0.077=0.09471 ampere-seconds. Assuming that thecharging current I_(c) applied to the battery cell 32 during the timeperiod between time t₂ and t₃ was measured to be 2.29 amperes, Q₆=2.29amperes×(t₃-t₂). Since Q₄+Q₅+Q₆ is assumed to be equal to 1.0 ampereseconds, the cut-off time period t₃ can be easily calculated to be0.1458 seconds. At time t₃,the FET Q14 is opened, thus disconnecting thecell 32 from the circuit. At time t₃, all three battery cells, 28, 30,and 32 have been charged to one ampere-second and have been disconnectedfrom the circuit. At time t₃, the charging current I_(d), supplied tothe battery cell 34 is measured. Since it was assumed that the powersupply 22 and regulator 24 can produce a maximum current of 4.0 amps,the charging current I_(d) to the battery cell 30 during this last timeperiod between t₄ and t₃ will likely be 4.0 amperes. In order todetermine the time period for disconnecting the battery cell 34 from thecircuit, the total charge for all of the time periods is added(Q₇+Q₈+Q₉+Q₁₀) is set equal to 1.0 ampere-seconds. The cut-off time t₄equals 0.629 seconds.

As mentioned above each charging cycle is assumed to be equivalent toone PWM cycle of the regulator 24. The above calculations are thus madefor every PWM cycle of the regulator 24 to thereby maintain arelatively-constant current for each of the battery cells 28, 30, 32,and 34, thus avoiding a thermal runaway condition that is normallyprevalent in such parallel battery chargers.

A flow diagram for the constant current operation is illustrated in FIG.5. As shown in FIG. 3B, each loop through the main program includes anoutput voltage and current adjustment in step 68. This currentadjustment is illustrated in detail in FIG. 5. Initially, in step 144,the charge (charging current x time) value for each pocket is set to apredetermined value. In the example discussed above, this value wasassumed to be 1.0 ampere-second. Next, in step 146, the FETS Q12, Q13,Q14, and Q15 (FIG. 1) are all turned on and the pocket number is set tothe number of pockets in the charger. In this example, four pockets areassumed. In step 148, the charging currents I_(a), I_(b), I_(c), I_(d)are measured for all four battery cells 28, 30, 32, and 34. As discussedabove, the turn-on time for the first cell 28 is calculated so that thecharge supply to that battery cell 28 is equal to the preset valuedetermined above in steps 150 and 152. Next, in step 154, the systemturns off the appropriate FET Q12, Q13, Q14, Q15 when the calculatedturn-off time has elapsed, as discussed above. Next, in step 156, thepocket number is decremented. The system next checks in step 158 if allof the pockets have been charged to the preset value determined in step158. If not, the system returns to step 148 and repeats the loop. If so,the system returns to step 144 and starts the process.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by a Letters Patent of theUnited States is:

1. A multiple cell battery charger comprising: a regulator for receivinga predetermined input voltage and supplying a regulated supply of DCvoltage at its output; a plurality of charging circuits, each chargingcircuit configured to charge an individual battery cell, said pluralityof charging circuits electrically coupled to said regulator andconnected in parallel with respect to each other, each charging circuitcomprising: a pair of terminals for coupling to a battery cell; aswitching device serially coupled to said pair of battery terminals forselectively connecting and disconnecting said pair of terminals fromsaid charging circuit forming a charging circuit; and a microprocessoroperatively coupled to said pair of terminals for periodicallymonitoring the current applied to each of said battery cells and thetime that such current is applied to said charging circuits andselectively controlling the switching devices to maintain a relativelyconstant charge on said charging circuits during a predetermined timeperiod.
 2. The multiple cell battery charger as recited in claim 1,wherein said regulator is a switching regulator with a selectable pulsewidth modulated (PWM) output signal.
 3. The multiple cell batterycharger as recited in claim 2, further including a sensing device forsensing the output current from the regulator, wherein said batterycharger is configured such that said selectable pulse width modulated(PWM) output signal is under the control of said microprocessor whichvaries the pulse width of said PWM output signal as a function of themagnitude of said the output current of said regulator forming a closedfeedback loop.
 4. The multiple cell battery charger as recited in claim1, wherein said predetermined input voltage to said regulator is AC. 5.The multiple cell battery charger as recited in claim 1, wherein saidpredetermined input voltage is DC.
 6. The multiple cell battery chargeras recited in claim 1, wherein said switching device is a field effecttransistor have gate, drain and source terminals. wherein said drain andsource terminals are serially coupled to said battery cell chargingcurrent sensing device and said battery cell terminals and said gateterminal is electrically coupled to said microprocessor.
 7. The multiplecell battery charger as recited in claim 1, further including a currentsensing device serially coupled to said pair of terminals for sensingthe charging current supplied to a battery cell.
 8. The multiple cellbattery charger as recited in claim 7, wherein current sensing deviceincludes a resistor.
 9. The multiple cell battery charger as recited inclaim 1, wherein said regulator is an integrated circuit.
 10. Themultiple cell battery charger as recited in claim 2, wherein saidpredetermined time period is a PWM cycle.
 11. A multiple cell batterycharger comprising: a regulator for receiving a predetermined inputvoltage and supplying a regulated supply of DC voltage at its output; aplurality of charging circuits, each charging circuit configured tocharge an individual battery cell, said plurality of charging circuitselectrically coupled to said regulator and connected in parallel withrespect to each other, each charging circuit comprising: a pair ofterminals for coupling to a battery cell; a switching device seriallycoupled to said pair of battery terminals for selectively connecting anddisconnecting said pair of terminals from said charging circuit forminga charging circuit; and a microprocessor operatively coupled to saidpair of terminals for periodically monitoring each of said battery cellsand selectively controlling the switching devices to maintain arelatively constant charge on said charging circuits during apredetermined time period in a constant current mode of operation . 12.The multiple cell battery charger as recited in claim 11 , wherein saidmicroprocessor also operates in a non-constant current mode ofoperation.
 13. The multiple cell battery charger as recited in claim 12,wherein said non-constant current mode of operation is a constantvoltage mode of operation.